1. Field of the Invention
This invention relates to improved liquid crystal display cells and more particularly to a method for treating the surface of a substrate to provide for selected alignment.
2. Description of the Prior Art
The alignment of liquid crystal molecules in liquid crystal devices is important for obtaining a high contrast ratio. The alignment of the molecules can be parallel to the cell substrate (homogeneous alignment) or it can be perpendicular to the cell substrate (homeotropic alignment).
The desired alignment can be obtained by doping the liquid crystal material with a dopant or, alternatively, by treating the surface of the cell substrate. The preferred approach is to treat the substrate surface in some manner. Various approaches to align liquid crystals by treating the cell substrate are described in the following patents: The patent to Jones et al., U.S. Pat. No. 3,904,797 bakes ionic surfactants onto the cell substrate. The patent to Sussman, U.S. Pat. No. 3,938,242 deposits a layer of metal on the substrate surface and subsequently oxidizes the metal layer. The patent to Harsch, U.S. Pat. No. 3,941,901 deposits a polymeric film on the cell substrate surface and then subjects it to a shear thinning technique. The patent to Kahn, U.S. Pat. No. 3,854,793 deposits a thin layer of a silane coupling agent on the substrate surface.